1. Field of Invention
This invention relates to an improved moving-iron instrument, such as an electrical meter.
2. Description of the Prior Art
A conventional moving-iron instrument, such as an electrical meter, comprises a field coil which is used to generate a magnetic field in response to an electric current applied thereto, and the interaction of a fixed iron piece, magnetized by the magnetic field, with a moving iron vane generates a driving torque so that a moving section comprising a pointer is rotated to indicate the quantity being measured.
FIG. 1 is a structural diagram of an important portion of a conventional moving-iron instrument comprising a moving section 1 comprising a moving shaft 1a, a pointer 1b, a damper 1c, a spring 1d, and a pair of moving iron vanes 1e and 1f, attached to shaft 1a; and a bottomed cylinder 2 of cylindrical shape, the side of an open portion being screwed to a frame 3. Cylinder 2 comprises fixed iron vanes 2a, 2b attached to the peripheral surface thereof so as to face opposite moving iron vanes 1e, 1f; a jeweled screw 2c screwed to a bottom part of cylinder 2; and a bearing or pivot (unnumbered) attached to the lower end of moving shaft 1a and coupled to jeweled screw 2c. The upper end of moving shaft 1a has a bearing or pivot (unnumbered) coupled to jeweled screw 2d as shown. An annular field coil 5 surrounds cylinder 2, and a bowl shaped sheild 6 surrounds coil 5.
In the moving instrument, field coil 5 generates a magnetic field in response to an electric current applied thereto. Interaction of fixed iron vanes 2a, 2b, magnetized by the magnetic field, with moving iron vanes 1e, 1f, generates a driving torque. As a result, the moving section 1 is rotated so that the resulting rotational position of moving section 1 can be read by observing the rotational position of the pointer 1b.
The conventional moving iron instrument of FIG. 1, however, has certain drawbacks, such as the following.
1. When the instrument is adjusted for sensitivity, or an extended scale is to be incorporated into the instrument, screws 3a, 3b are loosened and then cylinder 2 is rotated. This requires the rotating of screw 2c which is attached to cylinder 2. That is, even where the gaps between the bearing attached to the ends of moving shaft 12 and jeweled crew 2c, 2d have been once adjusted adequately, when the sensitivity adjustment is to be made or when the extension scale is to be incorporated, jeweled screw 2c, 2d must be loosened and cylinder 2 must be turned. This is time consuming, inefficient and subject to human error.
2. Cylinder 2 is usually made of a metal plate which is formed by a drawing process by a press. Jeweled screw 2c must be attached to the center of the bottom part of cylinder 2. Thus, since cylinder 2 is formed by the drawing process, it is difficult to located the exact center, and lower jeweled screw 2c is liable to deviate from the axial center of moving section 1. An off-center pointer leads to erroneous measurements.
3. Since cylinder 2 is made of metal, eddy currents flow. Consequently, an indication error of about -0.7% arises at 50 to 400 Hz. That is the conventional instrument of FIG. 1 has an inferior frequency characteristic.